Bicyclo pentenes homoantiaromaticity

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

Bicyclo[2.1.0]pentene, 116, can be considered to be the prototype of a neutral homoantiaromatic molecule. The types of structural and bonding effects found for this molecule parallel in many respects those found for the bicyclo[3.1.0]hexenyl cation reported above. Further studies on both of these 4q systems will likely be rewarding in terms of fully understanding the nature of cyclopropyl homoconjugation and homoantiaromaticity. 

Homoantiaromaticity is even less commonly invoked. Homocyclobutadiene 1.32b and the homocyclo-pentadienyl cation 1.33b are close to the transition structures for the interconversion of cyclopentadiene 1.32a and bicyclo[2.1.0]pentene 1.32c and of the cyclohexatrienyl cation 1.33a and the bicyclo[3.1.0]hex-enyl cation 1.33c. However, homoantiaromaticity does show up in these cases, in the sense that, unlike the interconversions in 1.30 and 1.31, neither of these interconversions is rapid. 

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